A significant component of industrial production involves web or sheet material movement generally linearly along conveyor systems. Materials undergoing these production procedures vary widely in density, chemical make-up, and the like, for example, from thin, transparent plastics to steel billets heated to incandescence. Speed of movement of these materials varies widely from the barely discernible to very fast, for example, temper mills in the steel industry often will convey sheet steel at speeds of about 90 mph. Essentially all such processes call for some form of guidance control. Widthwise measurement of moving sheet material often is called for in addition to a variety of position monitors. In the latter case, procedures for galvanizing or electroplating steel call for providing a continuous steel strip. Accordingly, successive sheet ends of this material are butt-welded together within an accumulator facility whereupon the welded unions are ground smooth and the resultant region of each union is marked for detection and subsequent removal by the positioning of a small hole in the sheet somewhere within its central region.
The environment under which sheet material conveyance is undertaken vary widely. Moving sheet webs are coated with photographic emulsions under highly refined environments designed to protect the halide chemistry of the coatings. Correspondingly, emergent steel typically is hot to the level of incandescence, for example about 3100.degree. F., such that position monitoring instrumentation must be supported far enough away to avoid its thermal destruction. In contrast, sheet aluminum production may involve, for example, a production environment of lower temperature but employing sprayed liquids including cutting fluids and kerosene which flow as foam and froth not only over the treated material but necessarily over monitoring instrumentation.
The development of position or dimension monitoring instrumentation for such processes has represented an elusive task for investigators. However, success has been achieved for certain of these environments and materials. A measurement system which has found substantial acceptance in industry is marketed under the trademark "SCAN-A-LINE". This system employs a linear array of light emitting diodes positioned on one side of a material such as a web or sheet moving within a production process. The diodes of this array are illuminated in a scanning sequence having a stable time base, for example, at a 20 KHz rate developed by a quartz crystal oscillator. Positioned above the moving material under production and opposite the associated diode array is a tuned photoresponsive receiver which reacts to the illumination emanating from those diodes which are unblocked or partially blocked at the edges of the moving material. The receiver and its associated controls then are called upon to carry out an extrapolation process to develop edge position. This extrapolation is based upon the observation that each LED in the emitting array produces a cone of light and the light cones from adjacent LEDs overlap each other in the light path to the receiver. An edge of the product being measured blocking the light path from the emitting diodes to the receiver will attenuate the light from more than one diode. The processing procedure carrying out extrapolation takes samples of the amplitude of the light received in sequence from the partially blocked and unblocked LEDs and develops a time-based stairstep light output pattern representing a scan across the edge which, in effect, is smoothed through the utilization of low pass filtering. The edge position of the material being observed then is defined as the time equivalent point on this smooth curve signal where the voltage drops to one-half of the peak LED signal amplitude. The "SCAN-A-LINE" system is marketed by Harris Instruments Corporation of Columbus, Ohio.
Harris, in application for U.S. patent Ser. No. 07/720,260 (supra) describes an improved "SCAN-A-LINE" system wherein each light emitting device of the array utilized is energized by a unique drive current which is pre-selected to cause the emission of light exhibiting substantially uniform intensity at the receiver when there is no attenuation of the light by the material under edge evaluation. Such balancing or optimization of the array light output not only achieves an importantly enhanced system accuracy in carrying out edge location, but it also substantially expands the range of application for such non-contacting measurement techniques. In this regard, the edge locating technique can be employed with transparent or semi-transparent materials. When so employed, the time-based trigger signal from which edge data is developed is generated at a location in scan time between a transition of detected amplitudes representing a maximum value and a minimum value. System accuracy is substantially improved additionally through the utilization of a receiving photo-detector assembly having a lengthwise dimension which is expanded. With the combination of this improved receiving approach and the balanced light values at the receiver, system performance has been observed to be improved beyond what would be expected.
A desirable aspect of the edge detecting and monitoring technique developed by Harris resides in its relatively lower cost as compared to other systems. Thus, where the approach can be expanded to successful utilization in different industrial environments and with different industrial materials, considerable advantage will accrue to industry. Obstacles facing the investigator looking to these environments are of numerous varieties. For example, where in-plant vehicles are utilized within the manufacturing environment carrying the sheet materials to be evaluated, those vehicles generally will have strobe lights mounted upon them in consequence of government mandated safety requirements. The strobe light is a device which broadcasts a spectrum which injects highly disruptive noise into the receiving components of edge monitoring systems. For example, should a strobe carrying vehicle be moved into adjacency with a monitor utilized to find the above-discussed holes indicating a union of two sheet components, the system generally will provide false data as to the presence of one of those hole indicators. Because of the spacing limitation between light source and receiver in the present system, they have been withheld from utilization in conjunction with the detection of heated materials such as steel ingots. Additionally, the infrared output form such materials when heated to incandescence would tend to disturb receiving components. Similarly, where the systems would be employed in environments where the emitters are impeded, for example, where they are continuously subject to foam and liquids, the resultant attenuation of light is so excessive as to render the light based edge detection monitors impractical.